Rochester Quadrajet issue

[transcorpiboats]

I have a 1982 23' I/O with a Mercruiser 260 and Alpha One I/O
I recently replaced the OEM carb (Rochester Quadrajet 4BBL) with a rebuilt one from National.Carburetor.
My mechanic has it working OK except for an issue with the secondaries. We suspect there is an adjustment needed there because when going from idle to WOT there is hesitation, flat spots and an intermittent "Knock", which occurs at higher rpm's (I'm attaching a video which outlines the problem. If you listen closely the intermittent "knock" can be heard. I'm more concerned with that than anything else and would like to know what it causing it.

http://www.transcorp.com/AutoIndex/HTML/IMG_1870.3gp

 

[alldodge]

Howdy

Unable to play the file but your symptoms of a lag, hesitation and knock all come from a lean condition. Your carb probably doesn't have the right size jets in it (to small), and the accelerator pump is not moving enough fuel during acceleration. Going from a Q-jet to another type can have its own issues. Did you replace with the same Q-jet or another type?

 

[Bt Doctur]

install a fuel pressure gauge and retest . Sounds like its going lean or there is an ignition issue

 

[transcorpiboats]

Thanks. As a footnote, I also notice the exhaust blacks up the back of my boat more than it used to...if it's too lean shouldn't there be less black stuff in the output?

 

[alldodge]

...if it's too lean shouldn't there be less black stuff in the output?

That's a Yes and a No

If the idle/primary jets are to large it will run rich at idle and a bit above.
If the secondary jets are to small it will run lean at higher rpms

 

[transcorpiboats]

OK. I'm still concerned about the "intermittent "knocking" sound at the higher rpm and if that could cause any damage.
Again, here's a video where that can be heard:

http://www.transcorp.com/AutoIndex/HTML/IMG_1870.3gp

 

[Bondo]

Ayuh,.... Welcome Aboard,... If ya wanta post a video, post it on you-tube or another public hosting site,....

Nobody wants to download a file not knowin' what it is, from whatever transcorp is,....

At this point in time, I'm seein' you as a spammer, rather than a Boater with a problem,....

 

[alldodge]

Did some looking into on the file type 3gp, and this comes from a Iphone media type. It can be seen when passed to another Iphone or if a file is downloaded to watch. Since it won't play with the standard media player I also would suggest uploading to youtube.

 

[transcorpiboats]

Actually I uploaded the file from my iphone to my company website for viewing because I couldn't attach it in the body of my initial post.
It will play using Quicktime.
I'm a boater with a problem looking for a little help from this forum.

 

[alldodge]

I'm willing to help but don't have quick time, have MS media player

 

[Bondo]

to my company website for viewing

Well, yer company server posted a link, that was a redirect to yet another site, which was changed to an iboats link by a Mod,.....

I don't do iphones, so I can't help ya,....

I still suggest ya use a public hostin' site, as If We, the folks that can help ya, don't feel comfortable, yer not gonna get much help,...

 

[transcorpiboats]

Thanks. On your recommendation, I uploaded it to youtube:

https://youtu.be/9qSTvK6KxnM

You will be able to hear that knocking towards the end of the video.

 

[alldodge]

That works, I hear the knock every so often. My first guess is your running lean (no enough fuel) and that's the knock. The way to confirm is let it start to knock, then slow and stop. Then pull a couple plugs out and my guess is they will be white. If it is lean, then the motor is destroying it self

 

[NHGuy]

It also seems to not be getting full rpm's. Did it run right before? And was it on the same propeller?
Sometimes if the prop has too high of a pitch the engine can't get up to 4500 rpm. In that case you might have knock that's being caused by too much load on the motor.
But if it previously ran well with that prop look at the replacement carburetor as perhaps being mis-jetted.

 

[muc]

Might be time to adjust your "air windup spring"
It's covered in the service manual or Google "Quadrajet air windup spring"

 

[jimmbo]

Thanks. As a footnote, I also notice the exhaust blacks up the back of my boat more than it used to...if it's too lean shouldn't there be less black stuff in the output?

If it so lean it will misfire and send unburnt fuel into the exhaust where it will partially burn from the other exhaust, and/or residual fuel from one or two misfires will eventually ignite, possibly rich.

 

[jimmbo]

The Q-jet is a highly engineered carb and is very versatile in its range of calibration. Replacing an OEM carb with a rebuilt, or even a new one, is always a 'Pig in a Poke' when it comes to calibration. Comparing jets and metering rods between the OEM and the replacement is still not a way to ensure the same fuel calibration. Air Bleeds, pullover circuits, secondary air valve preload, secondary metering rod hanger height, Primary metering rod piston spring. The Q-jet was made in two sizes, rated 750 cfm and 800 cfm, but very few will ever get to flow that much.
Before you go adjusting the secondary air valve preload, make sure the air valve is free to rotate and the secondary meter rods rise as the valve is opened. Next, are the secondary throttle valves opening? There is a lockout to prevent them from opening while the choke is on even the slightest. The primaries will open fully even if the secondaries are locked as the linkage is spring loaded. Another thing that can be a problem on the Q-jet is the power piston sticking in its bore. Operation can be check by carefully inserting a flat blade screwdriver that doesn't flare wide near the tip, into the bowl vent. angled towards the rear of the carb it should make contact with the top of the power piston and have a springy nature(be careful you can wreck this if not careful, otherwise the top of the carb has to come off.

 

[transcorpiboats]

I will check the plugs which will hopefully tell me if it is running too lean. Will also check the air wind-up spring.


I suspect the rebuilt carb does not have the proper jets, or the secondaries need to be adjusted. National has told me they will take this carb back under warranty, so I'm going to take them up on it. Hopefully the replacement will be right.

The engine runs almost perfect below 1500 rpms...I'm only getting this issue attempting WOT 3000+. I've only tried to run it that high a few times and have always backed off when I heard the knocking. Can I expect any internal engine damage just from the few attempts I've made here?

 

[transcorpiboats]

Also, thanks very much Jimmbo for your input (sorry I didn't see your reply earlier).
Looks like you have a great deal of experience with the Q-jet carbs.

 

[transcorpiboats]

Would a bad ignition module also cause this?